In order to cope with rapidly increasing energy consumption and change it into environment-friendly consumption, many researches have been conducted focusing on alternative energy and an alternative power source, i.e., an electrochemical energy production method. For storage and conversion of electrochemical energy, secondary battery, fuel cells, and capacitors are used. Especially, many researches upon lithium secondary batteries, which are known to have the most outstanding discharge performance, are being conducted.
Along with semiconductors and displays, secondary battery is one of top three core strategic product, which is expected to lead domestic industry of electronic information devices. As for future mobile IT products, such as cellular phones, notebook computers, camcorders, MP3 and PDA, closely related to twenty-first century human life, their performance relies on the secondary battery. Further, the secondary battery is getting more important as a power source of an electric vehicle.
Of the secondary batteries, a lithium polymer battery has been most frequently researched due to its high energy density and discharge voltage. Currently, the lithium polymer is being commercially used for mobile phones and camcorders.
With respect to electrolytes used for the lithium polymer battery, polyethylene oxide) (PEO)-based polymer electrolyte is currently known as one of polymer electrolytes having the highest possibility of commercialization. However, the polymer electrolyte using PEO exhibits relatively high ion conductivity of about 10−4 S/cm at a high temperature of 60° C. or higher, whereas it exhibits low ion conductivity of about 10−8 S/cm at a room temperature. This problem is attributed to high crystallinity (x=˜80%) of the PEO at the room temperature. Movement of ions within electrolyte is caused by segmentation movement of a polymer, and such movement is restricted in a crystalline area. In this regard, in order to develop polymer electrolyte having high ion conductivity and mechanical strength even at a relatively low temperature and the room temperature by suppressing the crystalline orientation of the polymer electrolyte, many researches are being conducted.
A conventional solid polymer electrolyte for a lithium secondary battery uses various additives for the purpose of controlling the crystallinity of PEO, which is a polymer matrix, in order to secure ion conductivity at a room temperature. For example, Korean Patent No. 10-0722834 describes “Preparing method of polymer electrolyte composite materials and lithium polymer battery using solid polymer electrolyte composite materials prepared by the method.” In most cases, however, upon introducing the additives, mechanical properties of the composition may be deteriorated, though crystallinity thereof may be controlled. Furthermore, since the sizes of the additives themselves affect mobility of PEO chains, the additives may cause an increase of a glass transition temperature (Tg), which is a highly important factor for ion conductivity at a room temperature and a low temperature. In addition, if the additives are introduced in order to improve strength of the solid polymer electrolyte, the strength of the solid polymer electrolyte may be improved at the expense of elongation. Accordingly, an additive capable of improving both the strength and the elongation of the solid polymer electrolyte needs to be developed.